Field of the Invention
The present invention concerns improvements in or relating to fibre-optic couplers.
Fiber-optic couplers are required in many fiber communication systems and have application for example as wavelength multiplexers/demultiplexers. In the latter application good wavelength selectivity is highly desirable.
Description of Related Art
The taper-twist fiber coupler for monomode fibers is well known (See, for example, Burres J. et al., Applied Optics 22 (1983) p. 1918). Such a coupler 10 is illustrated in FIG. 1. Here a pair of nominally identical monomode fibres 11 and 12 have been intertwined and, to form the coupler 10, these are stretched under tension and fused together. The intertwining assists merely to bring the two fibres 11 and 12 together to allow effective contact and fusion over a predetermined portion L of their length.
Such devices are modestly wavelength-selective as normally made as a result of their coupling properties. These effects are well-understood and can be described by the equations: EQU P.sub.1 =1-F.sup.2 Sin.sup.2 (Cz/F); EQU P.sub.2 =F.sup.2 Sin.sup.2 (Cz/F);
where P.sub.1, P.sub.2 are the powers flowing in fibers 11, 12 as a function of distance Z along the fibres, and it is assumed that initially
P.sub.1 =1, P.sub.2 =0;
C represents the coupling constant, a measure of the degree to which the fields are coupled, and the function F is defined by the relation: ##EQU1## where .DELTA..beta. is the difference between the propagation constants of the modes in the two fibres (without coupling).
The above formulae are not exact but are very good for practical purposes.
It is a widespread requirement that the coupler behave differently at different optical wavelengths (for example, for wavelength multiplexing etc.). To some extent this will occur anyway because the coupling constant C is approximately inversely proportional to wavelength. This produces an oscillatory behaviour of P.sub.1, P.sub.2 as the wavelength is varied. If z is large (i.e. C/F&gt;&gt;2.pi.) this variation with wavelength can be very rapid. However, it gives a near-sinusoidal wavelength response that is less useful than a response giving, for example, coupling only at or about a given wavelength. This can, however, be achieved by using also a variation of the function F with wavelength, so that at one wavelength the propagation constant difference .DELTA..beta..about.0 giving F.about.1 but elsewhere .DELTA..beta. is large enough to make F small, hence reducing coupling. Since it gives a simple, convenient response this latter is the preferred technique for making wavelength selective couplers. Note that simultaneous control of C and F is necessary.
For the fiber case in particular taper-fused fibers have been made from identical fibers (F=1) by twisting the fibers together and heating them, pulling at the same time. The effect of this is to reduce the core diameter and cause the core mode to become a cladding mode. Further pulling increases the coupler `z`, giving the first kind of wavelength selective coupler (ie. C-dependant) described above.
In producing the second, more satisfactory, type of coupler (i.e. C and F dependant) in this way, a difficulty is encountered. Although good core mode couplers of this type can be made using dissimilar fibers, fused couplers use cladding modes and this causes problems. To see this, consider FIG. 2. This illustrates variation of the propagation constant .beta. with wavelength .lambda., and is plotted for two values of fibre diameter (a) 2a.sub.1, the larger and (b) 2a.sub.2.
Here the propagation constant .beta. will vary with wavelength .lambda. for the cladding modes and the manner of this variation can be changed by using fibers 11 and 12 of different diameter 2a.sub.1, 2a.sub.2. However, because the fibers 11 and 12 generally have claddings which are all made of the same material, (silica in most cases), the curves (a) and (b) in FIG. 2 do not touch except in the trivial case in which the fibers are identical (not shown). If the cladding of the smaller fiber (a.sub.2) were made of a special material of higher refractive index .eta..sub.2 than the normal index .eta., this would have the effect of displacing the lower curve (b) upwards so that the desired crossing is produced (See FIG. 3). However, this is inconvenient and may produce problems in splicing.